Dryer control

ABSTRACT

Particulate material, such as an olefin polymer, is passed through a heated dryer where it is contacted with a stream of gas. The flow of material to the dryer is controlled in response to a measurement of the temperature differential between the material removed from the dryer and the gas removed from the dryer.

United States Patent [151 3,672,070

Stacy et al. [4 1 June 27, 1972 [54] DRYER CONTROL Primary Examiner-Carroll B. Dority, Jr.

Attorney-Young and Quigg [72] Inventors: Galen D. Stacy; Raymond G. Rohltlng,

both of Bartlesville, Okla.

[73] Assignee: Phillips Petroleum Company 22 Filed: Sept. 1, 1970 [57] ABSTRACT Particulate material, such as an olefin polymer, is passed APPl' 68,642 through a heated dryer where it is contacted with a stream of gas. The flow of material to the dryer is controlled in response 52] U S 34/25 34/56 to a measurement of the temperature differential between the material removed from the dryer and the gas removed from [51] [Ill-CL..." the dryer. [58] Fleld olSearch ..34/56, 25

. [5 6] References Cited v UNITED STATES PATENTS 4 Claims 4 3,396,476 8/1968 Eaves ..34/25 WATER 2e 1e 24 n 30 1 3| 32 DRYER 4| 2e POLYMER 2s DISPERSION [23 LSK'MMER AT 4 SIYMER as STRIPPER P 43 34 i STEAM 2| 2a as i r l l i i i WATER 35 39 STEAM DRYER CONTROL In the production of solid polymers of olefins it is often desirable to recover the polymer by a steam stripping operation in order to.remove solvent which may be present in the reactor effluent. This results in the formation of wet polymer particles which must be dried before the polymer is passed to an extruder and pelleted. Drying of the polymer is often accomplished by the use of a steam heated tube type of rotary drum dryer through which may be passed a stream of inert gas to help remove evolved moisture. In order to operate the process in the most efficient manner it is desirable to pass polymer through the recovery system at the greatest possible rate while still maintaining satisfactory operating conditions. However, it is important that the interior temperatures of the dryer not be excessive in order to prevent polymer from melting and sticking to the tubes and other interior surfaces of the dryer. A system is provided in accordance with this invention for controlling the operation of the drying system to meet these objectives.

In this system, the temperature of dried material removed from the dryer is measured, as is the temperature of the moisture-containing gas which leaves the dryer. A first signal is established which is representative of the difference between the two measured temperatures. This signal is in turn employed to control the rate at which polymer is passed through the system. The control system can utilize conventional analog control elements or it can involve a computation of dryer capacity by means of computing elements, which can be either in the form of a digital or an analog computer. In one specific embodiment of this invention, the flow of heating medium to the dryer is measured to provide an additional input signal which is utilized in the computation.

In the accompanying drawing,

FIG. 1 is a schematic representation of a polymer recovery and drying system having control apparatus of this invention associated therewith.

FIG. 2 is a schematic representation of a second form of the control apparatus which utilizes a computer.

FIG. 3 is a schematic representation of a first embodiment of the computer-controller of FIG. 2.

FIG. 4 is a schematic representation of a second embodiment of the computer-controller of FIG. 2.

Referring now to the drawing in detail and to FIG. 1 in particular, there is shown a system which is adapted for use in the recovery and drying of solid polymers of olefins. A solution of polymer in hydrocarbon solvent is introduced through a conduit which has a constant displacement pump 11 therein. This solution can be obtained as described in U.S. Pat. No. 2,957,861, for example. Conduit l0 communicates with the inlet of an inverted generally U-shaped tube 12 which extends into a stripper vessel 13. Water is introduced through a conduit 14 which communicates with the inlet region of tube 12. Steam is introduced into stripper 13 through a conduit 15. Vapors are removed from the top of stripper 13 through a conduit 16 and passed to suitable separation equipment, not shown, wherein solvent contained in the solution is recovered. The rate of introduction of water through conduit 14 is adjusted by a temperature controller 17 which manipulates a valve 18 in conduit 14. The input signal to controller 17 is established by a temperature sensing element 19 associated with tube 12. Water is thus introduced at a rate sufficient to maintain a desired temperature at the inlet to stripper 13.

A polymer-water slurry is withdrawn from the bottom of stripper 13 through a conduit 21 which has a pump 22 therein. The withdrawn slurry is passed to a skimmer 23 which has an auger 24 mounted in the top thereof to remove polymer which floats to the surface of the water. The skimmer is provided with an overflow drain 25. Wet polymer removed by auger 24 is passed through a conduit 26 to the inlet of a dryer 27. Dryer 27 can be a rotary dryer which is provided with internal coils or tubes through which steam or other heating medium is circulated. Dry polymer is removed through a conduit 28. An inert gas is passed through the dryer to contact the heated polymer and to help remove evolved moisture. This inert gas is introduced into the dryer through a conduit 29 and is removed through a conduit 30.

Heating steam is introduced into the dryer through a conduit 31 and condensate is removed through a conduit 32. This steam can be obtained from a source which provides 40 pound steam, for example. Such steam is introduced through a conduit 33 which communicates with a desuperheating vessel 34. The steam is allowed to bubble through water in vessel 34 to provide lower temperature steam, such as 15 pound saturated steam, for example, in conduit 31. Water is introduced into vessel 34 through a conduit 35 at a rate sufficient to maintain a desired level within the vessel. This is accomplished by a level controller 36 which regulates a valve 37 in conduit 35. The flow of steam into vessel 34 is adjusted by a temperature controller 38 which regulates a valve 39 in response to a measurement of the temperature of the steam in conduit 31. Steam is thus introduced into the dryer at a desired temperature which is sufficiently low to prevent the polymer from melting within the dryer.

In accordance with a first embodiment of the control system of this invention, temperature sensing elements 40 and 41 are disposed in respective conduits 30 and 28. Signals from these temperature sensing elements are applied to a differential temperature transducer 43 which provides an output signal representative of the difference between the two measured temperatures. The temperature measured by transducer 41 is normally the higher temperature, due to the countercurrent nature of the dryer. This output signal is applied to the input of a differential temperature controller 44. Controller 44 is provided with a set point signal AT which is representative of a predetermined desired temperature difierential between the two measured temperatures. Controller 44 establishes an output signal which is responsive to any difference between the measured differential and the desired set point value. This signal is applied to the set point of a pressure controller 45. A conduit 46, which has a control valve 47 therein, extends between the outlet and the inlet of pump 1 1. The rate at which the polymer dispersion is introduced into stripper 13 can thus be controlled by the opening of valve 47. If the opening of the valve is increased, a greater amount of the polymer dispersion is recycled from the outlet to the inlet of the pump. A pressure sensing element 48 is disposed in conduit 10 downstream of pump 11. A signal from this element is applied to the input of pressure controller 45. The output signal from pressure controller 45 regulates valve 47.

It should be evident from the foregoing description that the differential temperature measurement is employed to control the rate of flow of polymer dispersion into stripper 13. The illustrated pressure controller is employed to advantage as a flow controller because of the difficulty that would normally be encountered if an attempt were made to measure the flow of a viscous polymer solution through conduit 10 by a conventional flow measuring device. The measurement of pressure by element 48 serves as an indirect measurement of the flow in the illustrated system.

In one specific embodiment of the polymer recovery and drying system, dry polymer is removed from dryer 27 at a temperature of about 200F. Inert gas is removed from dryer 27 at a temperature of the order of to 180F. As the dryer approaches its maximum load, the polymer product temperature tends to decrease and the gas effluent temperature tends to rise. It is important to maintain a positive differential between these two measured temperatures so that dry polymer is produced. The control system of this invention is based on such a differential measurement and thus permits the dryer to be operated at maximum possible throughput while still maintaining a preselected temperature differential, such as 20 to 25 in this example.

A second embodiment of the control system of this invention is illustrated schematically in FIG. 2. The signals from transducers 40, 41 and 48 are applied to respective inputs of a computer-controller 50. In one embodiment, these signals are combined so as to provide an output signal 51 which is applied to the set point of controller 45 of FIG. 1. In one embodiment of the system of FIG. 2, an additional input signal is supplied by a flow transducer 52, which signal is representative of the rate of flow of steam through conduit 33.

The first embodiment of computer-controller 50 is illustrated schematically in FIG. 3. The signals from transducers 40 and 41 are applied to the respective inputs of a subtracting device 53 wherein the signal from transducer 40 is subtracted from the signal from transducer 41. The output signal from device 53, which is representative of the measured AT, is applied to the input of a delay device 54. The AT signal is also applied to an alarm 82 which notifies the operator in the event that the measured differential temperature falls below a preset value. Delay 54 is utilized to compensate for the time lag in the system between pump 11 and the outlet of dryer 27. The output signal from delay 54 is applied to a second subtracting device 55, which receives a preset AT set point signal. The resulting error signal, which is representative of any difference between the desired set point and the measured AT, is applied to the input of a multiplier 56. Multiplier 56 serves to multiply the error signal by a preset gain signal G to provide a proportional control mode. The product is applied to the first input of a summing device 57. The output signal from subtractor 55 is applied to a subtractor 58 and to a delay device 59. The output signal from delay device 59 is subtracted from the first input to subtractor 58 to provide an output signal representative of the derivative mode of control. This signal is multiplied by a second gain signal G in a multiplier 60. The resulting product signal is applied to the second input of summing device 57. The summed signal is transmitted through a limit device 62 wherein the input signal is compared with both a maximum and a minimum set point signal. The input signal is transmitted as long as it is between these two set point values. If the input signal should fall below the minimum set point or rise above the maximum set point, such set point signal is transmitted instead of the input signal. This serves as a safety device to prevent the control signal from exceeding predetermined limits. The signal transmitted by limit device 62 is applied to a summing device 63 wherein it is added to the measured pressure signal from transducer 48. The resulting output signal 64 is applied as the set point to controller 45.

The controller of FIG. 3 thus employs both proportional and derivative modes of control and compensates for the time lag inherent in the process of FIG. 1.

A second embodiment of the computer-controller of FIG. 2 is illustrated schematically in FIG. 4. The computer of FIG. 4 is employed to compute the percentage of the capacity C) of the dryer which is being utilized. This quantity is calculated in accordance with the following equation:

c K K T,,T, K T, K,F, K where T,, is the measured temperature of the dry polymer, T, is the measured temperature of the gas from the dryer, F, is the measured rate of steam introduction, and K K K K and K are constants. These constants are established by routine tests to provide an equation which best describes the desired control.

An input signal from transducer 41 is applied through a delay device 65 to the first input of a multiplier 66. A reference signal K is applied to the second input of multiplier 66. The resulting product is applied to the first input of a summing device 67. Signals from transducers 41 and 40 are applied to a subtracting device 53', the output of which is applied to an alarm 55 and through a delay device 54' to the input of a second multiplier 68. A reference signal K is applied to the second input of multiplier 68. The resulting product signal is applied to the second input of summing device 67. The signal from transducer 40 is applied through a delay device 69 to the first input of a multiplier 70. The second input to multiplier 70 is a reference signal K The resulting product is applied to a summing device 71. A signal from flow transducer 52 is applied through a delay device 72 to the first input of a multiplier 73. The second input to multiplier 73 is a reference signal K The resulting product is applied to the second input of summing device 71.

A reference signal, representative of the desired percentage of the capacity of the dryer to be utilized, is applied to a subtracting device 75. Signal K is also applied to device 75. The output signals from summing devices 67 and 71 and signal K are all subtracted from this set point signal. The resulting difference signal is applied to the input of a multiplier 56'. The remainder of the apparatus illustrated in FIG. 4 is identical to that shown in FIG. 3 and corresponding elements are designated by like primed reference numerals.

While this invention has been described in conjunction with presently preferred embodiments, it obviously is not limited thereto.

What is claimed is:

1. In a system in which material to be dried is passed through a dryer, the dryer is heated by passing a heat exchange fluid therethrough, and a stream of gas is passed through the dryer to contact the material and remove evolved vapor; control apparatus comprising means to sense the temperature of the material removed from the dryer and the temperature of the gas stream removed from the dryer and establish a first signal representative of the difference between such sensed temperatures, means to sense the rate of flow of said heat exchange fluid and establish a second signal representative thereof, means to combine said first and second signals and establish a control signal, and means responsive to said control signal to regulate the rate at which the material is passed through the dryer to tend to maintain said control signal constant.

2. The control apparatus of claim 1 wherein said means to regulate includes means to compute the percentage of the capacity C) of the dryer being utilized in accordance with the equation C= K T, K-,.( T,,T,,) K T, K 1", K, where T, is the measured temperature of the material removed from the dryer, T, is the measured temperature of the gas stream removed from the dryer, F, is said second signal, and K K K K and K are constants, and wherein the rate at which material is passed through the dryer is controlled in response to said calculated percentage.

3. The system of claim 1 wherein the material to be dried flows through a first conduit prior to being introduced into the dryer, a constant displacement pump is positioned in said first conduit, and a second conduit having a valve therein is connected between the outlet and the inlet of said pump, and wherein said means to regulate comprises means to adjust the opening of said valve.

4. In a process in which material to be dried is passed through a dryer, the dryer is heated by passing a heat exchange fluid therethrough, and a stream of gas is passed through the dryer to contact the material and remove evolved vapor; a method of control which comprises measuring the temperatures of the material removed from the dryer and the stream of gas removed from the dryer and establishing a first signal representative of the difference between said measured temperatures, measuring the rate of flow of the heat exchange fluid and establishing a second signal representative thereof, combining the first and second signals to establish a control signal, and adjusting the rate at which the material is passed through the dryer in response to said established control signal to tend to maintain said control signal constant.

* i I! l 

1. In a system in which material to be dried is passed through a dryer, the dryer is heated by passing a heat exchange fluid therethrough, and a stream of gas is passed through the dryer to contact the material and remove evolved vapor; control apparatus comprising means to sense the temperature of the material removed from the dryer and the temperature of the gas stream removed from the dryer and establish a first signal representative of the difference between such sensed temperatures, means to sense the rate of flow of said heat exchange fluid and establish a second signal representative thereof, means to combine said first and second signals and establish a control signal, and means responsive to said control signal to regulate the rate at which the material is passed through the dryer to tend to maintain said control signal constant.
 2. The control apparatus of claim 1 wherein said means to regulate includes means to compute the percentage of the capacity (% C) of the dryer being utilized in accordance with the equation % C K1Tp + K2(Tp-Tg) + K3Tg + K4Fs + K5 where Tp is the measured temperature of the material removed from the dryer, Tg is the measured temperature of the gas stream removed from the dryer, Fs is said second signal, and K1, K2, K3, K4 and K5 are constants, and wherein the rate at which material is passed through the dryer is controlled in response to said calculated percentage.
 3. The system of claim 1 wherein the material to be dried flows through a first conduit prior to being introduced into the dryer, a constant displacement pump is positioned in said first conduit, and a second conduit having a valve therein is connected between the outlet and the inlet of said pump, and wherein said means to regulate comprises means to adjust the opening of said valve.
 4. In a process in which material to be dried is passed through a dryer, the dryer is heated by passing a heat exchange fluid therethrough, and a stream of gas is passed through the dryer to contact the material and remove evolved vapor; a method of control which comprises measuring the temperatures of the material removed from the dryer and the stream of gas removed from the dryer and establishing a first signal representative of the difference between said measured temperatures, measuring the rate of flow of the heat exchange fluid and establishing a second signal representative thereof, combining the first and second signals to establish a control signal, and adjusting the rate at which the material is passed through the dryer in response to said established control signal to tend to maintain said control signal constant. 